1. Field of the Invention
The present invention relates to a liquid crystal display device in which a twist-aligned retardation plate is arranged with a liquid crystal cell using a nematic liquid crystal.
2. Description of the Related Art
Liquid crystal display devices are extensively used as displays for office automation equipments. Such a display device is required to provide a high-definition display and hence requires a large number of pixels and high time-division driving, and also requires, as display characteristics, a high contrast and a wide view angle. To meet these requirements, as display devices for personal computers and the like, a simple matrix type STN (super twisted nematic) liquid crystal display device or an active matrix type TFT-TN (thin film transistor-twisted nematic) liquid crystal display device, which can be driven in a high time-division manner and has a relatively high contrast among other liquid crystal display devices, is generally used.
The simple matrix type STN liquid crystal display device comprises a pair of substrates arranged to oppose each other with a predetermined distance between them, electrodes arranged on the opposing inner surfaces of the pair of substrates such that they cross each other at a right angle, orientation films formed to cover the electrode formation surfaces to orient liquid crystal molecules in a predetermined direction, a liquid crystal material sealed between these orientation films, and a pair of polarizing plates arranged outside the pair of substrates so as to sandwich them. In the liquid crystal material sealed between the pair of orientation films, liquid crystal molecules near the orientation films are arranged in a predetermined orientation direction by the orientation control force of the orientation films; that is, the arrangement of the liquid crystal molecules is twist-aligned at an angle around 240.degree. in a direction from one substrate to the other.
The active matrix type TFT-TN liquid crystal display device comprises a twisted nematic (TN) liquid crystal cell. That is, a common electrode is formed on one substrate, and scan lines and signal lines are formed to cross each other on the other substrate. A pixel electrode and a thin-film transistor (TFT) for driving are arranged at each intersection of these lines. A liquid crystal material in which the arrangement of liquid crystal molecules is twist-aligned at an angle of approximately 80.degree. to 120.degree. is sealed between the pair of substrates.
These liquid crystal display devices are driven in a time-division driving manner, changing the orientation of liquid crystal molecules in accordance with an electric field applied between the opposing electrodes, and controlling transmission and interruption of light by the optical action of the liquid crystal layer sandwiched between the pair of polarizing plates, thereby presenting a desired display.
In the STN liquid crystal display device, however, the twist angle of the arrangement of liquid crystal molecules is increased to allow high time-division driving, and the effect of birefringence of liquid crystal is used to increase the visual contrast, resulting in a problem of coloring of a display. In addition, the viewing angle is not sufficiently wide, and displayed colors change depending on the visual angle.
The TFT-TN liquid crystal display device, on the other hand, can be driven by applying a static voltage to each pixel and therefore has a higher contrast and a wider viewing angle than those of the simple matrix type liquid crystal display device. In displaying halftones in a multi-gradation-level display, however, the reversal of brightness occurs depending on the visual angle, disturbing the multi-gradation-level display.
FIGS. 1A to 1D show equi-Y value (value of brightness) curves for four gradation levels in a conventional TFT-TN liquid crystal display device. In a liquid crystal cell of this liquid crystal display device, the arrangement of liquid crystal molecules is twist-aligned counterclockwise through 90.degree. in the direction of propagation of light, a product .DELTA.n.sub.c.d.sub.c of a refractive index anisotropy .DELTA.n.sub.c and a gap d.sub.c is 403 nm, a ratio d.sub.c /p.sub.c of the gap d.sub.c to a natural pitch p.sub.c is 0.05, and a pretilt angle is 3.degree.. Voltages for realizing the four gradation levels are a bright-state voltage of 1.5 V, a dark-state voltage of 6.0 V, and two halftone voltages which correspond to two intermediate Y values obtained by equally dividing the Y value upon application of 1.5 V into three portions; that is, in the ascending order of voltage, first-gradation-level voltage V1=1.50 [V], second-gradation-level voltage V2=2.06 [V], third-gradation-level voltage V3=2.43 [V], and fourth-gradation-level voltage V4=6.0 [V]. Referring to FIGS. 1A to 1D, concentric circles represent visual angles tilted by 10.degree., 20.degree., 30.degree., 40.degree., and 50.degree., from the inside, with respect to the direction of normal to the substrates of the liquid crystal display device. A closed square (.box-solid.) represents a Y value of 10, an open square (.quadrature.) represents a Y value of 20, and a closed triangle (.tangle-solidup.) represents a Y value of 30. An arrow R indicates the orientation direction on the substrate on the light incident side of a liquid crystal cell. The azimuth of the display surface is represented by an angle (to be referred to as an angle of azimuth hereinafter) measured about the center of the display surface with reference to the light-incident side orientation direction R. As an example, the angle of azimuth in the upward direction on the display surface is 135.degree.. As is apparent from FIGS. 1A to 1D, the upper portion is bright, and the lower portion is dark, at any gradation-level voltage.
FIGS. 2A to 2C illustrate equi-contrast curves (Y1/Y2, Y2/Y3, and Y3/Y4) each obtained by the Y value ratio of neighboring gradation levels. Referring to FIGS. 2A to 2C, concentric circles represent visual angles tilted by 10.degree., 20.degree., 30.degree., 40.degree., and 50.degree., from the inside, with respect to the direction of normal to the substrates of the liquid crystal display device. A closed circle (.circle-solid.) represents a contrast of 1 or less, a closed square (.box-solid.) represents a contrast of 10, an open square (.quadrature.) represents a contrast of 20, a closed triangle (.tangle-solidup.) represents a Y value of 50, and an open triangle (.increment.) represents a contrast of 100. As can been seen from FIGS. 2B and 2C, regions (hatched portions in FIGS. 2B and 2C, which will be referred to as halftone reversal regions hereinafter) with a contrast of 1 or less appear between the second and the third gradation levels and between the third and the fourth gradation levels. The halftone reversal region in the upper portion of FIG. 2B is produced because the third gradation level is brighter than the second gradation level. The halftone reversal regions in the lower right and left portions of FIG. 2C are produced because a leakage of light occurs at the fourth-gradation-level voltage. This reversal of brightness in a halftone display disturbs a correct gradation display and hence is a serious problem of the liquid crystal display device.